This invention relates to battery power systems and more particularly relates to a battery isolation or protection circuit for a multi-battery power system of the type commonly used on special purpose motor vehicles that are equipped with tools, appliances or other electrically powered equipment that require more than the single battery that is conventional on a motor vehicle. The invention is capable of protecting the multiple batteries when the batteries are of different types, such as when the auxiliary battery is a gel cell battery, and also reduces the heat dissipated by the protection circuit and its cost.
Vehicle manufacturers design and build a variety of general purpose vehicles with an electrical system that includes a chassis battery for starting the engine and supplying power for operating the electrical loads that are the vehicle accessories, such as its lights and electronic equipment. These electrical components are typically installed by the manufacturer on the vehicle chassis because they are necessary for operating the vehicle or desired by vehicle owners for typical uses of the vehicle. A flooded cell, lead-acid battery is the industry standard chassis battery. An electrical power generator that is driven by the engine is also included with the chassis components for charging the chassis battery and supplying electrical power to the chassis loads. In most modern vehicles the electrical power generator is an alternator although it can alternatively be a DC generator or other device that generates electrical power for charging batteries and for operating electrical loads as well as the associated electrical devices that are commonly installed by vehicle manufacturers. The electrical power generator and the chassis battery have terminals connected to a common ground on the vehicle and ungrounded terminals that are electrically connected together so that those components are electrically connected in parallel. The electrical power generator has its own electronic control that is capable of varying the power generator's output voltage as a function of one or more sensed inputs.
However, some vehicles are additionally equipped with special purpose electrical equipment which must also be supplied with electrical power. Examples are utility work vehicles, emergency vehicles and motor homes. These vehicles are often manufactured by specialized companies which purchase a chassis from vehicle manufacturers and outfit the chassis with added equipment. Because the special purpose equipment usually has a high power demand, the vehicles are provided with an auxiliary battery so that the vehicle has both a chassis battery and an auxiliary battery. The auxiliary battery has a terminal connected to the common ground and can supply electrical power to operate auxiliary loads such as power tools, medical equipment or appliances in a motor home. The auxiliary battery avoids the need to run the engine and its alternator to power the auxiliary loads and maintain the chassis battery in a charged state. Operating the engine of a motor home disturbs neighbors, especially at night. Additionally, in some locations, commercial vehicles that arrive at a work site are required to stop running their engines and are prohibited from running generators in order to prevent exhaust pollution, reduce noise and conserve fuel. Because the auxiliary loads need power from somewhere, an auxiliary battery is installed.
Unfortunately, many vehicle owners treat their auxiliary batteries carelessly. They leave electrical loads on when not being used, even when the vehicle is parked overnight. Such neglect discharges the auxiliary battery, often to a completely discharged state.
The prior art developed protection circuits known as battery isolators, battery separator switches or auxiliary battery disconnect switches. Because it is undesirable to modify the standard chassis wiring of a vehicle, protection circuits are confined to connections to battery terminals and the vehicle's common ground. The protection circuit must meet several operating needs. These include: (1) at times connecting the vehicle's electrical power generator to the auxiliary battery so it can charge the auxiliary battery as needed but without altering the charging of the chassis battery in the manner designed by the vehicle manufacturer, (2) at times disconnecting the auxiliary loads from the auxiliary battery if the auxiliary battery becomes excessively discharged and (3) at times separating the auxiliary battery from the chassis battery in order to prevent the auxiliary battery and its auxiliary loads from discharging the chassis battery and rendering it unable to start the vehicle engine.
FIG. 1 is an illustration of two prior art battery protection circuits that are drawn similarly to drawings of the invention in order to make clear both the similarities and important differences between the circuitry of the prior art and the circuitry of the present invention. A vehicle from a vehicle manufacturer has a chassis electrical power system 8 that comprises a chassis battery B1, an electrical power generator 10, typically an alternator, and a chassis load L1 (a composite of the multiple individual chassis loads). All have a terminal connected to a common ground 12 and an ungrounded terminal 14 electrically connected together so they are in parallel electrical connection. Chassis battery charging and the supply of power to the chassis loads is independently controlled by the alternator control system and switches included in the chassis electrical system by the vehicle's manufacturer. An auxiliary battery B2 has a grounded terminal 16 and an ungrounded terminal 17. An electrically controlled battery separator switch 18 is interposed in direct electrical connection between the ungrounded chassis battery terminal 14 of the chassis battery power system 8 and the ungrounded auxiliary battery terminal 17. An auxiliary load L2 has a grounded terminal 20 and an ungrounded terminal 22 that is connected through an electrically controlled switch 24 to the ungrounded terminal 17 of the auxiliary battery B2.
The auxiliary battery often consists of a bank of several parallel connected batteries. The chassis battery may also be more than one battery. The chassis load and the auxiliary load usually consist of several individual electrical loads. For simplicity, the singular terms “load” and “battery” and single symbols are used to represent the equivalent composite of the multiple loads and batteries that are commonly installed on vehicles.
The switch 18 is controlled by an auxiliary battery charge control circuit 26 that switches the switch 18 to an open or closed state. The auxiliary battery charge control circuit 26 includes a voltage sensing circuit and the switch 18 is switched as a function of the chassis battery B1 voltage V1 which is identical to the voltage of the power generator 10 because they are connected in parallel. Switch 18 is closed and connects the auxiliary battery B2 directly to the vehicle's power generator 10 and battery B1 when the voltage V1 at terminal 14 is sufficiently above the fully charged state of the chassis battery, for example 13.2 volts, so that the power generator 10 will charge the auxiliary battery B2 and, if switch 24 is closed, also supply power to the auxiliary load L2. Switch 18 is opened when the power generator 10 voltage V1 falls to a level, for example 12.8 volts, that indicates that the chassis battery B1 is being charged, in order to maintain the chassis battery B1 in a fully charged state that is capable of starting the engine.
It is important to note that the first switch 18 of the prior art circuit is controlled only by the sensed value of the voltage V1 at the terminal 14 of the chassis electrical power system 8. The fact that the first switch 18 is closed for a sensed voltage V1 that is greater than some fully charged voltage (for example 13.2 volts) means that switch 18 is also closed for larger voltages. Consequently, the highest voltage that the vehicle's voltage regulator causes the power generator 10 to have is also applied to the auxiliary battery. As will be seen from subsequent description, in modern vehicles the power generator 10 can reach voltages that damage or destroy a battery if it is a gel battery. But the prior art circuit does not protect against that problem.
The switch 24 is controlled by an auxiliary battery discharge control circuit 28 that switches the switch 24 to an open or closed state. The auxiliary battery discharge control circuit 28 includes a voltage sensing circuit and the switch 24 is switched as a function of the auxiliary battery B2 voltage V2. Switch 24 is closed to connect the auxiliary battery B2 to the auxiliary load L2 when the voltage of the auxiliary battery B2 is high enough, for example greater than 11 volts. Switch 24 is opened when the voltage V2, for example 10 volts, of the auxiliary battery B2 indicates that it is discharged and therefore further discharge should be prevented.
These prior art circuits perform well, are reliable and provide a long battery lifetime when both the chassis battery and the auxiliary battery are flooded cell batteries, which have essentially the same electrical characteristics. However, this prior art circuit has some deficiencies.
One deficiency is that the switch 18 requires two series connected MOS/FETs. It requires two series MOS/FETs because switch 18 must be able to block current flow in both directions when switch 18 is open. If current could flow through the MOS/FETs in one or the other direction when switch 18 is open, either the auxiliary battery B2 and/or the auxiliary load L2 could discharge the chassis battery B1 or the chassis electrical power system 8 could discharge the auxiliary battery B2. Because every MOS/FET has an inherent diode between its source and drain, for example the diodes 30 and 32, if switch 18 were a single MOS/FET, a discharging current could flow from one battery to the other through the inherent diode. The requirement for two MOS/FETs has two important consequences for this prior art circuit. Although MOS/FETs that are designed for switching high currents are relatively low resistance devices, these circuits operate with very high currents that result in considerable heat dissipation in each MOS/FET making heat sinks necessary. As will be seen in the description of the invention, the improved circuit of the invention requires only one MOS/FET in each of its two switches. One consequence is that the circuit of the invention reduces the heat dissipation, and resulting energy waste, by 66% when current is supplied by the vehicle power generator to the auxiliary load. The second consequence is that the circuit cost is reduced by the elimination of one MOS/FET.
In addition to the above-described deficiencies with the prior art circuits, an additional problem has arisen because of relatively recent developments in battery technology that better meet the needs of an auxiliary battery but that also create problems for prior art battery protection circuits. The development is the gel battery. Before its development, the standard storage battery for decades has been the flooded cell, lead acid battery which has a liquid electrolyte and a solid physical separator to mechanically hold the electrodes in separated positions. The gel battery, more specifically the gelled electrolyte lead acid battery, has its electrolyte retained in a gel. A further development has been the advanced glass mat (AGM) lead acid battery in which the separator is essentially a sponge-like glass mat. Additional developments can be expected in the future to which the present invention is applicable.
The gel battery has numerous advantages. The chassis battery and the auxiliary battery must meet different demand conditions which make their electrical requirements different. The chassis battery needs to be capable of providing a very high current for a relatively short time period in order to start the vehicle. It also needs to supply the relatively low current demand for operating the vehicle accessories. The auxiliary battery needs to be a deep cycle battery that can supply a large current for a long period of time and can withstand numerous repetitive deep cycle discharges and recharges.
Deep cycle flooded cell batteries have long been used as auxiliary batteries. But flooded cell batteries are capable of a relatively limited number of deep cycles, for example about 250 deep cycles. Gel batteries are capable of a far greater number of deep cycle discharges and recharges, for example 700 deep cycles. Because of this extended capability, gel batteries are now used for auxiliary batteries sometimes in a group of ten on some commercial vehicles. However, because flooded cell batteries exhibit characteristics that make them more suitable for use as chassis batteries, gel batteries are not used as chassis batteries. The result is that vehicles are now being equipped with two different types of batteries with different physical, electrical and chemical properties. Typically the chassis battery is a flooded cell battery and the auxiliary battery is a gel battery. Although the prior art battery protection circuits have adequately protected vehicles that use flooded cell batteries for both the chassis battery and the auxiliary battery, they do not adequately protect vehicles with the two different battery types.
Although gel batteries have the capability for better performance and longer service lifetimes as auxiliary batteries than the flooded cell batteries, they have not reached their potential because they are protected by prior art battery protection circuits which provide inadequate protection of gel batteries. Consequently, the experience of users in the field has been that gel auxiliary batteries have exhibited a useful lifetime and performance that are considerably less than their capabilities.
Therefore, it is an object and feature of the present invention to provide a battery protection circuit that can adequately protect both a gel battery used as an auxiliary battery when connected to a vehicle alternator and a vehicle flooded cell battery, despite the difference in the electrical characteristics of the two batteries.
It is a further object and feature of the invention to provide a battery protection circuit in which the heat dissipation generated by the electrically controlled switches, such as MOS/FETs, is reduced by two thirds and therefore dissipating only one third the power dissipated by the prior art protection circuits.
It is a further object and feature of the invention to provide a battery protection circuit of reduced cost by elimination of a circuit component that has been necessary in the prior art battery protection circuits.